As an air filter for a vehicle, there has been known a wet-type filter having filter paper impregnated with oil. Viscose oil is generally used as impregnation oil. Accordingly, it is called the “viscose type” air filter. The viscose type air filter has an advantage of extended lifetime in comparison with the dry-type filter, which is not impregnated with oil. In the viscose type air filter, oil on the surface of the filter captures dust and then the oil penetrates into the captured dust so as to provide a function of capturing the other dust on the surface of the captured dust. Such a chain capture of dust permits to increase an amount of captured dust per volume of the filter.
Japanese Utility Model Publication No. S63-27767 describes the improved viscose type air filter. The air filter described in Japanese Utility Model Publication No. S63-27767 is a laminate of the first filter material having a relatively high density and the second filter material having a relatively low density. Such a laminate structure permits to make oil impregnation ratios in the first and second filter materials different from each other, resulting in prevention of clogging of the filter, providing an effective capture of dust.
The wet-type filter impregnated with oil has a problem that carbon particles cannot be captured effectively. The dry-type filter, which is not impregnated with oil, can capture effectively carbon, but has a problem of a decreased amount of captured dust per volume of the filter. Use of non-woven fabric leads may cause problems that dust permeation occurs at a high flow rate and in a serious pulsation and costs increase.